1. Field of the Invention
The subject invention relates to a utility vehicle which is self propelled with its path controlled by a central processing unit to follow a predetermined route, and more specifically, to a lawn mower.
2. Description of the Prior Art
The prior art includes various systems wherein a lawn mower or harvester includes sensors to position itself in an area. A distance measuring system employing ultrasonic waves is disclosed in U.S. Pat. No. 4,733,355. An electro-optical and microcomputer for automatic guidance is disclosed in U.S. Pat. No. 4,482,960. A system employing a combination of optical and radio frequency signals to determine the velocity of a vehicle is disclsosed in U.S. Pat. No. 5,239,296.
While the basic idea of measuring the time of propagation of the signal to and from a locating station is old, the implementation is complicated when a high degree of accuracy is required. In other words, the period of the counter, i.e. the clock speed that drives the counter, directly limits the accuracy for determining the position of a mower.
Practical counters can be implemented at speeds up to 5 GHz. This implies a basic count period of 200 picoseconds. Radio energy travels at the speed of light or about 300 million meters per second. In 200 pico seconds, a radio signal will travel about 6 cm. This accuracy is not sufficient for the mower which requires accuracy of 0.5 cm or better. Furthermore, while 5 GHz counters are the outside limit of available performance, 2 GHz counters are more practical. A 2 GHz counter reduces the accuracy to 15 cm, clearly unacceptable.